(i) Field of the Invention
This invention relates to a method for measuring thermal diffusivity of material, an apparatus used for this purpose, and a method for measuring thermal conductivity. Particularly, it relates to a measuring method and apparatus depending on the non-steady state method (where the temperature is not kept constant and hence is allowed to vary) for precise measurements of the thermal diffusivity of a hardly conductive material such as high molecular compounds (polymers) and ceramics, and to a method for measuring its thermal conductivity by the use of the measured value of the thermal diffusivity obtained according to the method for measuring thermal diffusivity.
(ii) Description of the Background Art
Thermal diffusivity and thermal conductivity are among important properties for determining the processing and use conditions of a variety of materials including high molecular compounds (polymers) upon their material and product designs. Recently, a number of various simulation programs have been developed keeping pace with the advance of computerization, and material and product designs making use of these programs have been put into practice so often. For instance, structural analysis for analyzing the stress or deformation of processed products or structures and thermal conduction analysis for analyzing heat transfer phenomena have already been applied widely. Heat flow analysis for analyzing the behavior of resins in a mold in the injection molding has recently been utilized in many cases. The analytical accuracy of these simulation programs is largely dependent on the accuracy of the physical property values used in the analyses, not to mention the contents of the programs. Accordingly, in order to improve their analytical accuracy and perform precise material and product designs, it has been desired to measure the physical properties of the object material with high precision.
Processed products are often used practically not only at room temperature but also at high temperatures. While processing, most of high molecular materials undergo a molding process in which they are molten at high temperatures and then cooled to room temperature. Therefore, when material and product designs are carried out taking into account the actual use and processing conditions of a product or when analysis is conducted on the basis of actual phenomena, it is necessary to know its physical properties in the wide range of temperature from room temperature to the melting temperature or above.
Lately, it has frequently been practiced to use processing materials in a composite form, so that their combinations have become diversified and complicated. On measuring physical properties for the material development and design of such a particular processing material, it has often been difficult to obtain a large amount of its sample. It has also become necessary to know its physical properties as quickly as possible and reflect the results without delay in the development and design of its product. As a result, rapid measurement of the physical properties has been demanded with a small amount of sample.
The method for measuring thermal diffusivity is broadly divided into the steady state method and the non-steady state method. The method for measuring thermal diffusivity according to the non-steady state method is characterized in that the thermal diffusivity is determined by producing forcibly a thermally non-equilibrium state in a sample and measuring the temperature variation of the sample caused by the subsequent relaxation of the state. Thus, the method has such an advantage over the steady state method that measuring time is significantly reduced.
Typical methods for measuring thermal diffusivity according to the conventional non-steady state method include the Angstrom method, the flash method and the PAS method. The Angstrom method calculates thermal diffusivity in the following manner: A portion of a rod sample, which has a sufficiently small cross sectional area relative to the length, is brought into contact with a heat source which causes periodical heating and cooling, whereby a periodic temperature variation is produced on the end of the sample and, in consequence, a temperature wave is brought about in the sample. The state of propagating the temperature wave in the sample is observed by measuring the temperature at not less than two measuring points which are different in distance from the heating point in the propagation direction of the wave. Using the amplitude and phase of the temperature wave obtained at each measuring point, the thermal diffusivity is calculated.
The flash method measures thermal diffusivity in the following manner: One of the surfaces of a plate sample is provided with a light absorption layer, to which a laser pulse, for example, is irradiated to effect instantaneous heating through light absorption. The temperature rise thereby caused at the absorption layer is propagated in the thickness direction of the sample so that a temperature variation is produced on the surface of the sample opposite to the irradiated surface. The temperature variation is measured as a function of time after the flash irradiation. From the temperature-time curve thus obtained, the thermal diffusivity is determined.
The PAS method determines thermal diffusivity in the following manner: A closed cell having a window for transmitting light is equipped with a microphone, etc. for measuring sound signal. A plate sample within the cell is provided with a light absorption layer at one surface thereof. A modulated light beam is irradiated through the window to the layer to produce a periodic temperature variation. This temperature wave is propagated to cause a periodic temperature variation on the opposite surface of the sample. Then, the variation of a periodic sound wave thereby generated in the cell is measured. Using the phase and amplitude of the wave, the thermal diffusivity is determined.
The conventional measuring methods described above involve the following problems.
In the Angstrom method, since the sample has to be molded into a long rod, a great amount of sample material is required, and the installation of its adiabatic system becomes large in order to minimize the heat loss from the surface of the sample. Further, the measurement requires a relatively long time and hence the objects to be measured are limited to materials of relatively large thermal diffusivities.
Since the flash method effects heating through light absorption, when a transparent sample or a sample of little light absorption is measured, it is necessary to coat an absorption layer for light absorption on the surface of the sample. This often causes a heat loss or heating irregularity at the interface of the absorption layer and the sample, which are responsible for errors. Further, the measurement is carried out on the assumption that no consideration of heat loss may be made because of short time. This assumption is well met by those with large thermal diffusivities such as metals. However, with those of smaller thermal diffusivities such as high molecular compounds, large errors will result.
In the PAS method, the same problems as in the flash method arise owing to the heating through light absorption. Further, since the method measures sound signal by means of a sound signal detector, it is affected seriously by noises caused by vibration, sound, etc.
Moreover, it is difficult for these measuring methods to measure the temperature-dependence of thermal conductivity. This measurement in actual practice necessitates an elaborate apparatus.
It is the objects of the present invention to solve the foregoing problems, and to provide a method for measuring thermal diffusivity, according to which precise measurements can be made even with materials of small thermal diffusivities, measurements can be made with trace amounts of samples, and rapid measurements including the temperature-dependence can be made in a small scale apparatus, an apparatus thereof, and a method for measuring thermal conductivity according to which the thermal conductivity is determined from the measured value of the thermal diffusivity.